I figured it out! SO If you download the base game & the DLC's from Steam, download them and then click on the games little start up bit on steam (where you can click to play the game, see your time spent on the game etc)
Look next to the game title at the right of the screen and you'll see a settings button - click it then go to 'manage', followed by 'view local files' - from there, come back on here and download the adult patch, then just transfer it over to the folder with the game files and voila!